uucore: set meaningless FsUsage.ffree value to 0
uucore: set meaningless FsUsage.ffree value to 0

Set the value of the `FsUsage.ffree` value to 0 on Windows, because
even though it is meaningless, it should not exceed the
`FsUsage.files` value so that client code can rely on the guarantee
that `FsUsage.ffree <= FsUsage.files`.

uutils/coreutils,uutils/coreutils,uutils/coreutils